Make OCR optional

[manthey]

I'd like to make having the ocr library optional, as it adds some installation burden and we'd like to use the rest of this plugin in other projects. Specifically, make easyocr an extra_requires in setup.py (as [ocr]).

Perhaps we should also add a value to the girder.local.conf file that sets the state of whether the ocr job should be run on import IF that the setting in the conf file exists (otherwise it would leave the state of the run-on-import setting alone).

In the ideal, if easyocr isn't installed, we'd show a note that OCR is unavailable in the settings page and not show its buttons. This would try to import easyocr and flag if it isn't available.